Radio frequency identification (RFID) technology is commonly used for storing and transmitting information associated with a specific object. RFID technology utilizes a tag transponder, which is placed on the object, and a reader, also referred to herein as an interrogator or transmitter/receiver, to energize, read and identify the tag. RFID technologies are broadly categorized as using either “active” tags or “passive” tags. Active tags have a local power source (such as a battery) so that the active tag sends a signal to be read by the interrogator. Active tags have a longer signal range. “Passive” tags, in contrast, have no internal power source. Instead, passive tags derive both processing and transmitting power from the transmitter, and re-transmits information back to the reader. Existing technologies for passive tags typically have a much shorter signal range (typically less than 200 feet).
Both categories of tags have electronic circuits that are typically in the form of an integrated circuit or transistor array based on silicon or metal oxide “chip” technology. The circuit stores and communicates identification and other data to the reader. In addition to the chip, the tag includes an antenna that is directly connected to the chip. Active tags incorporate an antenna that communicates with the reader from the tag's own power source. For passive tags, the antenna also acts as a transducer to convert radio frequency (RF) energy originating from the reader to electrical power. The chip then becomes energized and performs the processing and communication function with the reader.
“Nonactive” component RFID tags or just nonactive tags (sometimes referred to as “chipless” tags) are inherently passive and operate without using any active electronic components or semiconducting materials including integrated circuit(s) or any active discrete electronic components, such as the transistors or diodes. This feature allows nonactive tags to be printed directly onto a substrate at much lower costs than traditional RFID tags using widely available bar code printing technologies such as spray or screen printing.
As a practical matter, RFID in general, and specifically nonactive tag technologies use lower radio frequencies which have much better materials penetration characteristics, will work under more hostile environmental conditions, and do not require geometrical alignment (i.e. do not need a direct line-of-sight between tag and reader) compared to bar code reading technologies. Therefore, nonactive tags may be read through paint, water, dirt, dust, human bodies, concrete, or through the tagged item itself. Similar to traditional RFID tags, nonactive tags may be used in managing inventory, automatic identification of cars on toil roads, security systems, electronic access cards, keyless entry and in reporting environmental conditions.
The principle element of RFID tags that are typically prepared via stamping/etching techniques is the RF antenna, where a foil master is carved away to create the final structure with specified frequency response. The RFID antenna may also be printed directly on the substrate using a conductive metal or polymer ink. The ink is printed on a substrate, followed by high temperature sintering used to anneal the particles and to create a conductive pathway or line on the substrate. Alternatively, metal fibers may be incorporated directly into the substrate. Although particulate metal materials may be used, the superior characteristics of nanoparticle metal materials suspended in conductive organic inks results in a better product. Metallic nanoparticles are particles having a diameter in the submicron size range. Nanoparticle metals have unique properties, which differ from those of bulk and atomic species. Metallic nanoparticles are characterized by enhanced reactivity of the surface atoms, high electric conductivity, and unique optical properties. For example, nanoparticles have a lower melting point than bulk metal, and a lower sintering temperature than that of bulk metal. The unique properties of metal nanoparticles result from their distinct electronic structure and from their extremely large surface area and high percentage of surface atoms.
Metallic nanoparticles are either crystalline or amorphous materials. They can be composed of pure metal, such as silver, gold, copper, etc., or a mixture of metals, such as alloys, or core of one or more metals such as copper covered by a shell of one or more other metals such as gold or silver. The nozzles in an inkjet printing head can be less than 1 μm in diameter. In order to jet a stream of particles through a nozzle, the particles' size should be less than approximately one-tenth of the nozzle diameter. This means that in order to inkjet a particle, its diameter must be less than about 100 nm.
Nickel or iron particles have been used for conductive inks for a very limited extent because of its relatively low conductivity (approximately four times less than that of copper). However in nonactive tags such magnetic materials may be required for enhanced inductor performance. Gold and silver can provide good electrical conductivity without magnetic effects, but are relatively expensive. Moreover, gold and silver require high temperatures for annealing, which can pose a challenge for printing on paper and plastic substrates. Copper provides good conductivity at a low price (about one percent of that of silver). Unfortunately, copper is easily oxidized and the oxide is non-conductive.
Copper-based nanoparticle inks are unstable and require an inert/reducing atmosphere during preparation and annealing in order to prevent spontaneous oxidation to non-conductive CuO or Cu2O. Copper polymer thick film (PFT) inks have been available for many years and can be used for special purposes, for example, where solderability is required. Another interesting strategy is to combine the advantages of both silver and copper. Silver plated copper particles are commercially available, and are used in some commercially available inks. Silver plating provides the advantages of silver for inter-particle contacts, while using the cheaper conductive metal (copper) for the bulk of the particle material. Thus, the preferred reliable means of preparing copper antennae is via electroplating on an existing metal surface.
No current technology exists for an inexpensive bio-organic or organic/metal particle composite based RFID nonactive tag structure that permits identification and environmental parameters to be associated with an item.
A printed antenna with passive and non-active discrete components or an entirely printed circuit including the antenna and the passive components are two paths towards the inexpensive production of high quality nonactive RFID tags. However, because RFID tags do have internal digital electronic circuitry, the capacity of nonactive RFID tags to store large amounts of data is limited. Nevertheless, nonactive RFID tags are an ideal vehicle for use in measurement systems involving multiple items with requiring tags that are very inexpensive, simple to produce, environmentally friendly and biodegradable. Thus there is a need for a tag structure, such as nonactive RFID tags, that provides item identification and environmental information.